circleci: Move to CircleCI 2.1 format.

This contains the minimum changes required to move to the CircleCI 2.1
configuration file format. In 2.1 job names should not contain "." in
them.

Moving to 2.1 will allow us to enable enable pipelines from CircleCI
project settings UI. Once pipeline is enabled auto-cancellation of
redundant workflows can be also enabled from the CircleCI UI.

See https://circleci.com/docs/2.0/build-processing/

.circleci/config.yml

@@ -3,7 +3,7 @@
 # See CircleCI upstream's docs on this config format:
 #   https://circleci.com/docs/2.0/language-python/
 #
-version: 2
+version: 2.1
 aliases:
   - &create_cache_directories
     run:
@@ -82,9 +82,10 @@ aliases:
          || echo "Error in uploading coverage reports to codecov.io."
 
 jobs:
-  "trusty-python-3.4":
+  "trusty-backend-frontend-py34":
     docker:
       # This is built from tools/circleci/images/trusty/Dockerfile .
+      # Trusty ships with Python 3.4.
       - image: gregprice/circleci:trusty-python-5.test
 
     working_directory: ~/zulip
@@ -107,9 +108,10 @@ jobs:
       #     # also /tmp/zulip-test-event-log/
       #     destination: test-reports
 
-  "xenial-python-3.5":
+  "xenial-backend-py35":
     docker:
       # This is built from tools/circleci/images/xenial/Dockerfile .
+      # Xenial ships with Python 3.5.
       - image: gregprice/circleci:xenial-python-4.test
 
     working_directory: ~/zulip
@@ -125,9 +127,10 @@ jobs:
       - *run_backend_tests
       - *upload_coverage_report
 
-  "bionic-python-3.6":
+  "bionic-backend-py36":
     docker:
       # This is built from tools/circleci/images/bionic/Dockerfile .
+      # Bionic ships with Python 3.6.
       - image: gregprice/circleci:bionic-python-1.test
 
     working_directory: ~/zulip
@@ -156,6 +159,6 @@ workflows:
   version: 2
   build:
     jobs:
-      - "trusty-python-3.4"
-      - "xenial-python-3.5"
-      - "bionic-python-3.6"
+      - "trusty-backend-frontend-py34"
+      - "xenial-backend-py35"
+      - "bionic-backend-py36"